Ecological and taxonomic differences between rare and common plants of southwestern Ontario

We compared ecological attributes in rare and common native plants of southern Ontario (n = 1,398 flowering plant species; of these, 375 are identified as provincially rare). Compared with patterns for common species, rare species were significantly (P < 0.05) over-represented in one taxonomic order (Fabales) and two families (Boraginaceae and Fabaceae) and significantly under- represented in two orders (Alismatales and Saxifragales) and one family (Salicaceae). Rare species were significantly more likely than common species to be associated with open-habitat communities (tallgrass prairie, alvar, and meadow) and significantly less likely to be found in aquatic habitats. Rare species were also significantly more likely to be insect-pollinated and to have larger fruits than common species. Furthermore, they were less likely to be dioecious, wind-pollinated, shrubby, clonal, or to produce fruits having many seeds. Rare-common differences were examined across nine large, ordinal-level monophyletic groups in order to reduce the phylogenetic influences of derived traits. Results concerning pollination, fruit size, number of seeds per fruit, life form, and clonality were each confirmed within at least one monophyletic group, suggesting that over-abundance of derived traits did not produce these rare- common results. In a separate comparison of rare woody species, rare species were significantly more likely (than common woody ones) to have a short flowering period, animals as dispersers, and large fruits; rare woody species were less likely to be wind dispersed or to inhabit moist/wet substrates. An inability to exploit new habitats or to cope with anthropogenic change appear to be the most important general features associated with rarity. These results support previous conclusions that no single characteristic can reliably predict which species are or will become rare.